Tide

The periodic rise and fall of sea level driven by the moon and sun, which controls depth, currents, and visibility at many coastal dive sites.

Tides are the regular rise and fall of sea level caused by the gravitational pull of the moon and sun. Most coastlines see two high and two low tides a day, roughly 6 hours 12 minutes apart, with the range between them varying from centimeters in the Mediterranean to over 10 m in places like the Bay of Fundy. For divers, the tide sets the water depth, but more importantly it drives tidal currents.

As the tide floods in or ebbs out, water accelerates through channels, around headlands, and over shallow reefs. Many sites are only diveable, or only pleasant, in a specific window around high or low water when the flow eases. Incoming tides often bring clean oceanic water and better visibility, while outgoing tides can drag silty inshore water across a site.

Serious shore and drift divers plan around tide tables the way pilots plan around weather. Note the tidal state on each dive: after a season of entries you will know exactly which hour of the cycle gives your local site its best conditions. Apps and printed tables both work, just make sure they reference the station nearest your actual site.